So I'm 6 brews in (4 all grain) now and have figured out a bunch of my mistakes, but the US vs UK gallon thing still confuses me no end.
Carboys are 6 US gallon, 5 Imperial, and I know that most recipes list in US gallons, but why is it that a lot of recipes don't use the full volume of the carboy/primary? I thought it was bad to leave a lot of airspace over the beer, or is that a misconception too?

It is. You want plenty of head space for primary fermentation or you will have a mess on your hands when that krausen shhoots all over your ceiling.
I do my primary in 6-1/2 gallon and 8 gallon buckets.
"IF" you do a secondary, then you want limited head space

It is. You want plenty of head space for primary fermentation or you will have a mess on your hands when that krausen shhoots all over your ceiling.
I do my primary in 6-1/2 gallon and 8 gallon buckets.
"IF" you do a secondary, then you want limited head space

This. And when you have active fermentation happening, the headspace will soon fill with co2, pushing the air out.

Wow, I've never caught until now that an imperial gallon is not part of the imperial measurement system, but is in fact a metric measurement. So the US uses imperial measurements, but not imperial gallons. Everybody else uses metric measurements, and imperial gallons. Who thought of that.

Didn't mean to thread jack, definitely +1 on the extra head space comments.

There are no gallons in the metric system. Only liters are used to measure liquid. I think the imperial gallon is the way they used to measure liquid, but not since I've been educated. From a 32 year old Canadian.

Overall it's ridiculous and I hate it either which way, but I force myself to just concede and use all US standards, it's just easier. Except Liters. How Liters somehow remains so highly used in a dominantly US forum, when a quart is practically the same thing....but again, it's all nuts.

Would that we could all just have a single way of measuring, but then what would all the calculators do, right?